The present invention relates to a fuel pump.
Japanese Laid-Open Patent Publication No. 2009-209838 discloses a fuel pump that pressurizes and discharges a fuel pumped up by a feed pump from a fuel tank. The fuel pump disclosed in this document includes a pump housing having a cylinder and a columnar plunger located inside the cylinder. One end portion of the plunger is inserted in an interior of the cylinder, and the other end portion projects externally from the cylinder. In this state, the plunger is located to slide in a reciprocating manner along its central axis. On the end portion of the plunger projecting externally from the cylinder, a groove extending circumferentially along an outer peripheral surface of the plunger is formed. A holed disk-shaped plate is engaged with the groove of the plunger. A cam reciprocating the plunger in the interior of the cylinder is located under the fuel pump. A lifter is located between the cam and the end portion of the plunger projecting externally from the cylinder. Between a lower portion of the fuel pump and the plate, a coil spring to press the plate against an upper surface of the lifter is located.
FIG. 17A shows an example of a coil spring to be installed in the fuel pump described above. As shown in FIG. 17A, in the vicinity of end portions of the coil spring, the coil spring has a smaller winding interval toward the terminal end of the winding. In the coil spring described above, elasticity against compression varies depending on the position around a central axis L of the coil spring. For example, the modulus of longitudinal elasticity in a left-hand part X where the winding interval is relatively wide in the vicinity of the end portions is smaller than that of a right-hand part Y where the winding interval is relatively narrow. Therefore, as shown in FIG. 17B, when a compressive force Fc is applied to the coil spring, the coil spring contracts further in the part X than in the part Y. Therefore, the coil spring curves in a manner such that a central part bulges to the right. As a result, rightward acting side force Fs is generated in the coil spring.
In the above-described conventional fuel pump, when such a side force as above is generated in the coil spring, the side force is transmitted to the plunger via the plate. At this time, a moment acts on the plunger in a direction inclined with respect to the cylinder. When the plunger inclines due to the moment, friction that occurs on a sliding surface between the plunger and cylinder locally increases, causing possible abnormalities in wear or heat generation in the plunger or cylinder.